The ISP is rather realistic about at their HelpDesk unwillingness to support the full zoo of home (amateur) devices. They have named their output network as "lan" ::) and expect to see an average home setup with no segmentation, with all PC's, WiFis and IoTs bound together :-[ .

To satisfy the more demanding clients, the CPE firmware started to support Prefix Delegation last year. Client prefix is /56, Inteno effectively halving that for its service LAN (resulting /57 prefix) but for some technical and political reasons prefix /62 is offered to the client devices, if asked nicely. There are different firmware versions in the field, some of these permit manually changing the prefix size (tested all possibilities from 57 through 64).

With my network and adm experience, I have spent tens of hours if not more, reading blogs and forums and RFCs. So far no light in the end of tunnel, thus asking the community now.